In the related art, it is known that a Focused Ion Beam (FIB) by a Transmission Electron Microscope (TEM) produces an observable thin sample. Furthermore, it is also known that a damage layer is formed by injecting a gallium a ionic species into the thin sample and irradiating the thin sample with a FIB.
In recent years, as a means for removing the damage layer, a finishing process has been proposed in which the thin sample is irradiated with a Gas Ion Beam (GIB) to remove the damage layer (see JP-2007-066710 A). According to such means, it is possible to form a thin sample having few damage layer.
However, in the means of the related art, if a structure such as a semiconductor device is exposed to an observation surface of the thin sample, since an etching rate of the GIB is varied by the presence or absence of the structure, a phenomenon in which irregularities are formed in the observation surface and become a streak, that is, a so-called curtain effect occurs. Thus, there is a problem that the streak formed by ion beam processing also appears in an observation image of the observation surface in addition to an original device structure.
Particularly, in a high technology device of recent years, various kinds of materials have been used. Thus, a difference in the etching rate may be remarkable.
Furthermore, since the structure of a high technology device is small, an observation object is also small. Thus, even if irregularities appearing in the observation surface are small, the irregularities affect the observation.